The Miller effect refers to the use of a capacitor present on a circuit board to simulate a capacitor having a higher value for a portion of the electronic circuit present on the board.
FIG. 1 shows a device DISP1 for feedback-controlling a voltage comprising a feedback control loop BA1 and a Miller compensation device according to the prior art.
The feedback control loop BM comprises an operational amplifier AO1 and the Miller compensation device comprises a capacitor C1. The device DISP1 supplies power to a load Zch1.
An input voltage Vin1 is applied to a negative input E11 of the amplifier AO1. A positive input E12 of the amplifier AO1 is linked to an output SR1 of a variable gain R1. A first terminal ER11 of the gain R1 is linked to a ground GND and a second terminal ER12 is linked to an output terminal Sout1 of the device DISP1. The load Zch1 is linked in parallel to the variable gain R1 to the ground GND and to the terminal Sout1.
The capacitor C1 is linked both to the terminal Sout1 and to an output SAO1 of the amplifier AO1. The capacitor C1 is known to those skilled in the art as a “Miller capacitance” and is sized such that the device DISP1 is stable within the bandwidth of the amplifier AO1. The gate G1 of a PMOS transistor T1 is linked to the output SAO1, the drain D1 of the transistor T1 is linked to the terminal Sout1 and the source S1 of the transistor T1 is linked to the potential Vcc.
When a DC voltage is applied, the output voltage Vout1 of the device DISP1 at the terminal Sout1 is equal to the value of the variable gain R1 multiplied by the value of the voltage Vin1. The value of the variable gain R1 is chosen such that the value of the voltage Vout1 is equal to a predetermined constant value, for example 5 V.
The potential at the gate G1 is equal to Vcc minus the gate-source voltage VGST1 of the transistor T1. The value of the voltage VGST1 is for example equal to 0.7 volts.
The device DISP1 controls the output voltage of the amplifier AO1 such that the potential at the output SAO1 is substantially equal to the potential Vcc, the voltage at the output of the amplifier AO1 is equal to Vcc minus the voltage VGST1.
The potential Vcc may reach high values, for example a value of 28 volts.
The potential Vcc is for example the positive terminal of a battery for an automobile, the voltage of which varies between 6 and 28 volts.
Consequently, the value of the voltage across the terminals of the capacitor C1 may reach high values, for example up to 22.3 volts, and damage the capacitor C1, which generally withstands a maximum value of 3.64 volts.
FIG. 2 shows the gain curve G1 and the phase curve φ1 according to frequency F for a capacitor C1 of 200 pF of the device DISP1.
A voltage feedback control device DISP2, comprising two operational amplifiers AO2 and AO3 that is suitable for high voltages and supplies power to a load Zch2, is known from the prior art.
The device DISP2 is shown in FIG. 3.
The voltage Vin2 is applied to an input E21 of the amplifier AO2 and an input E31 of the amplifier AO3. The inputs E21 and E31 are linked to the negative terminals of the amplifiers AO2 and AO3, respectively.
Inputs E22 and E32 that are linked to the positive terminals of the amplifiers AO2 and AO3, respectively, are linked to an output SR2 of a variable gain R2. A first terminal ER21 of the gain R2 is linked to a first terminal of a switchable gain R3 and a second terminal ER22 of the gain R2 is linked to an output terminal Sout2 of the device DISP2. A second terminal of the switchable gain R3 is linked to a ground GND. The load Zch2 is linked in parallel to the variable R2 and switchable R3 gains, to the ground GND and to the terminal Sout2.
The gate G2 of a PMOS transistor T2 is linked to an output SAO2 of the amplifier AO2, the drain D2 of the transistor T2 is linked to the terminal Sout2 and the source S2 of the transistor T2 is linked to the potential Vcc.
The gate G3 of a PMOS transistor T3 is linked to an output SAO3 of the amplifier AO3, the drain D3 of the transistor T3 is linked to the terminal Sout2 and the source S3 of the transistor T3 is linked to the potential Vcc.
A current Iout2 flows at the output of the device DISP2 through the terminal Sout2 and creates a voltage Vout2 across the terminals of the load Zch2.
In this example, the amplifiers AO2 and AO3 are sized for two different load current Iout2 value ranges such that at least one amplifier is stable within one of the two load current value ranges. The amplifier AO2 is activated when the current Iout2 is smaller than, for example, 20 mA, and the amplifier AO3 is activated when the current Iout2 is larger than 20 mA in this example. The consumption of the amplifier AO2 is of the order of 15 μA and that of the amplifier AO3 is of the order of 2 mA.
Depending on the value of the current Iout2, the switchable gain is activated and one of the amplifiers is activated, or in other words if the value of the current Iout2 is higher than 20 mA, the gain R3 is activated and the amplifier AO3 is activated. If the value of the current Iout2 is lower than 20 mA, the gain R3 is deactivated and the amplifier AO2 is activated.
This device no longer includes a capacitor.
FIG. 4 shows the output voltage Vout2 of the device Iout2 according to the current Iout2 at the output terminal Sout2. Variations in the curve according to the load current Iout2 may be seen.
However, this device has the drawback of increasing the consumption of the device when the amplifier AO3 is activated, and requires an additional device for detecting the value of the current Iout2, of the value of the potential Vcc and for monitoring the switchable gain R3.
Moreover, when one amplifier is switched to another, a discontinuity in the output voltage of the device is observed. In the case of a window lift motor controlled by a microprocessor, the output voltage Vout2 supplies power to the microprocessor. Variations in its supply voltage may hinder its operation, leading to the window lift operating poorly.
Consequently, there is a need to modify a compensation device known from the prior art such that it withstands high potentials, has low consumption, is simple to implement and exhibits a near-linear output voltage.